Re: Legacy stuff isn't the problem.
There are several separate issues here.
One is that the system is currently earning money. Where's the pay for all those dev[op]s working on shiny new systems coming from? Probably from that despised legacy system.
Then there's the age of the system and its state. There are a couple of assumptions being made. One is that it's old and the second is that, simply because it's old it's ill-maintained. Neither is necessarily true.
For one thing there was an article on here a little while ago ( https://www.theregister.co.uk/2016/06/16/online_graze_in_reverse ) about one relatively new business deciding its whizzy web-based system no longer suits and has to be replaced. This isn't stuff conceived in COBOL on a 360 but it's still legacy.
It's also by no means certain that something that was first put together years ago hasn't been maintained properly in the interim. If it's running the main line of business there's every reason to make sure its fit for purpose. If it isn't then its maintainers haven't been doing their jobs right.
The problem with attitudes expressed here, and maybe with legacy systems if they're not well maintained is that new development is seen as important, challenging, rewarding and whatnot, able to adopt the latest buzz word methodologies of agile and devops. Maintenance is just maintenance and, if done right, involves serious thinking about how to graft new stuff in seamlessly and keep documentation up-to-date. Actually it can be more challenging and rewarding if done right but it's apt to be seen as somewhere where those least able to do it well get pushed out of the way. Which is exactly the wrong way to treat what's paying everyone's wages.